The research, funded by the Biotechnology and Biological Sciences Research Council
(BBSRC) and carried out by scientists at the UCL Institute of Opthalmology and the University of
Tromsø, Norway, showed that the colour change helps reindeer to see better in
the continuous daylight of summer and continuous darkness of Arctic winters, by
changing the sensitivity of the retina to light.

Arctic reindeer, like many animals, have a layer of tissue
in the eye called the tapetum lucidum (TL) which lies behind the retina and
reflects light back through it to enhance night vision.

By changing its colour, the TL reflects different wavelengths
of light.

In the bright light of summer the TL in Arctic reindeer is
gold, similar to many other mammals, which reflects most light back directly
through the retina. However by winter it has changed to a deep blue which
reflects less light out of the eye.

This colour change in reindeers' eyes gives them an advantage when it comes to spotting predators in the low light of an Arctic winter.

Professor Glen Jeffery, UCL Institue of Ophthalmology

This change scatters more light through photoreceptors at
the back of the eye, increasing the sensitivity of the retina in response to
the limited winter light.

The team believes this would be an advantage in the
prolonged murk of winter, allowing reindeer to more easily detect moving
predators and forage.

Lead researcher Professor Glen Jeffery (UCL Institute of Ophthalmology) says: “This
is the first time a colour change of this kind has been shown in mammals. By
changing the colour of the TL in the eye reindeer have flexibility to cope
better with the extreme differences between light levels in their habitat
between seasons.

“This gives them an advantage when it comes to spotting
predators, which could save their lives.”

The colour change may be caused by pressure within the eyes.
In winter, pressure in the reindeers’ eyes is increased, probably caused by
permanent pupil dilation, which prevents fluid in the eyeball from draining
naturally. This compresses the TL, reducing the space between collagen in the
tissue and thus reflecting the shorter wavelengths of the blue light common in
Arctic winters.

Previous work from Professor Jeffery and Norwegian
colleagues from Tromso had shown that Arctic reindeer eyes can also see
ultraviolet, which is abundant in Arctic light but invisible to humans, and
that they use this to find food and see predators. The blue reflection from the
winter eye is likely to favour ultra-violet sensitivity.